Storing Energy From Solar and Wind Isn't Always the Best Idea

By Dave Levitan

Posted 10 Sep 2013 | 19:12 GMT

Energy storage is often considered an ideal scenario when it comes to renewable energy. Saving up energy generated in bright and windy conditions and using it when the sun stops shining and the wind stops blowing feels like a no-brainer. But a new paper by researchers at Stanford University suggests that in certain circumstances, simply curtailing, or slowing down, that wind turbine can be a better deal—energetically speaking—than storing up the power.

"Curtailing renewable resources results in an immediate and obvious forfeiture of energy," the investigators wrote in the journal Energy & Environmental Science. They cite the example of Texas, where as much as 17.1 percent of wind generation was curtailed each year between 2007 and 2012; that totals a massive 13 terawatt-hours of electricity. "However, flexible grid technologies [including storage] can also consume significant amounts of energy in their manufacture and operation. These embodied energy costs are not as immediately apparent, but they are an energy sink from a societal perspective." In other words, storage technologies cost energy in various ways as well, and it is no guarantee that building and deploying them will represent energy savings in all scenarios.

The researchers calculated the energy return on investment (EROI) for various storage technologies in combination with solar and wind power. First of all, when it comes to solar, all technologies considered, including compressed air storage and batteries like zinc-bromine and vanadium redox, worked better than curtailing the generation. So, for solar: always store what you can, no matter what storage medium you've got lying around. The best energy returns come from compressed air and pumped hydroelectric, but lithium-ion batteries aren't bad either.

Wind power is another story. While compressed air and hydroelectric storage make sense, no battery technology out there is good enough to yield an EROI for which storage would beat out curtailment. Li-ion is closest, but still lags far behind compressed air and pumped hydro. The battery tech brings EROI down well below the curtailment level (see bottom graph), but interestingly, the wind-plus-battery combo still has a higher overall EROI than solar photovoltaic power does on its own; that's thanks to wind having a roughly10-fold better EROI than solar.

This means that purely from an energy perspective—and thus an emissions perspective as well—certain storage technologies are not worth pairing with a wind farm. Notably, this analysis does not take economics or other factors into account, but if energy is the point here, then batteries and wind are a no-go at the moment. The study's authors recommended that research should work on battery cycle life: increasing the number of cycles a battery is capable of by a factor of as much as 20, so they can handle 10 000 to 18 000 cycles. That level would bring the EROI numbers up to make curtailment the worse option. For the moment, though, this means that an increasing focus on compressed air in particular is probably the right move.